1. Field of the Invention
The present invention relates to an antenna apparatus, a radar and a waveguide that are used in high frequency bands such as a microwave band, an extremely high frequency (EHF) or millimeter wave band, etc.
2. Description of the Related Art
In the past, there has been known an antenna apparatus which includes a laminated body that has a waveguide portion formed of a plurality of laminates or plates laminated one over another, and an antenna element plate that is placed on the laminated body and emits or receives electromagnetic waves, wherein each of the laminated plates and the antenna element plate are coupled with each other through surface to surface contact.
In this case, when the peripheral portions of the laminated plates are fastened to one another by means of fastening members such as screws, rivets or the like, there are generated gaps between the individual laminated plates by the fastening forces of the fastening members, as a result of which the following defects will occur. That is, electromagnetic waves leak from gaps of the waveguide portions which have been formed by laminating the laminates or laminated plates one over another, so not only the transmission loss is increased but also the isolation between the waveguide portions is deteriorated.
In addition, there has been a problem that an excitation phase error is generated due to the leakage of the electromagnetic waves from the gaps of the waveguide portions, thus reducing the directional characteristic of the antenna.
In order to avoid the occurrence of such problems, it is necessary to eliminate the gaps between the laminated plates as much as possible thereby to ensure the conduction or propagation of electromagnetic waves between the laminated plates.
As a means for ensuring the conduction of electromagnetic waves between the laminated plates, there are described the bonding of laminated plates by means of electroconductive rubber in a first patent document (Japanese patent application laid-open No. H8-186401), or by means of an electroconductive adhesive material in a second patent document (Japanese patent application laid-open No. 2003-318641), or by means of an adhesive sheet or adhesive material in combination with bumps in a third patent document (Japanese patent application laid-open No. 2004-15579).
Moreover, a fourth patent document (Japanese patent application laid-open No. 2004-221718) describes a means for preventing leakage of a high frequency signal, even if there are generated gaps between laminated plates, by causing a spurious short-circuit of a choke structure at nearby or adjacent portions.
However, in the case of the means using the electroconductive rubber or the electroconductive adhesive material, the leakage loss of electromagnetic waves becomes large because the electric conductivities of the electroconductive rubber and the electroconductive adhesive material are poorer than those of metals. In addition to this, there has been another problem that the adhesive strengths of the electroconductive rubber and the electroconductive adhesive material are decreased due to the aging or change over time thereof, or change of the ambient temperature, or the like. In particular, in the case of the electroconductive adhesive material, there have been a working problem, such as how to coat the adhesive material, what amount thereof is required, etc., and a material management problem such as the difficulty in storing or preserving the adhesive material.
Further, in the case of the bump structure, there have been the following problems. That is, electric conduction can not be ensured at locations other than bump portions, and when the bump structure is used in the millimeter wave band, bumps can not be formed with respect to small-sized waveguides.
Furthermore, in the case of the choke structure, this is advantageous in terms of aging and working in the temperature characteristic, but on the other hand, there have been a problem that it is necessary to ensure appropriate places in which choke grooves can be formed, at locations close to the waveguides, and hence it is difficult to reduce the size or dimensions of the entire apparatus, and a problem that the formation of fine choke grooves should be achieved at a very high cost.